How do nMOS and pMOS transistors work in terms of signal strength

When you combine nMOS and pMOS transistors in parallel, you get a transmission gate. This gate can pass both 0s and 1s effectively, making it a better switch for signals.

A transmission gate needs both the control input and its complement, known as double rail logic. This makes sure that the signals are not degraded, which is crucial for reliable circuit operation.

When we look at nMOS and pMOS transistors, their behavior in handling signal strength comes down to their operating principles and how they respond to the applied voltages.

NMOS Transistors

nMOS devices are great for pulling a signal low. When you apply a high voltage to the gate of an nMOS transistor, it allows current to flow from the drain to the source, effectively grounding the output and driving the signal towards 0V (logic LOW). This makes nMOS transistors strong pull-down devices. However, when you try to use nMOS to pass a HIGH signal, it tends to degrade slightly due to the threshold voltage drop, which is why the output might not reach the full supply voltage.

PMOS Transistors

pMOS transistors, on the other hand, are excellent at pulling a signal high. When you apply a low voltage to the gate, it creates a conductive path for current to flow from the source to the drain, driving the output to the supply voltage (logic HIGH). This makes pMOS transistors strong pull-up devices. However, similar to nMOS, pMOS transistors are weaker at passing a LOW signal, as there can be a slight voltage drop.

Signal Strength in CMOS Logic

When we combine nMOS and pMOS in complementary pairs, as we do in CMOS technology, you get the best of both worlds. The nMOS transistor handles the pull-down operation efficiently, and the pMOS transistor handles the pull-up operation. This ensures strong signal levels for both HIGH and LOW states without significant degradation, which is why CMOS circuits are so reliable and widely used.